At least once a week I forget to turn off the lights to my plants overnight. Therefore, they get a 36 hour photo period.
Can this harm my plants?
In the short run no, but in the long run, yes. Plants do need down time when their is no sunlight. They use use lightless nights to process the sugars they make during the day. Also, light inconsistency can throw temperate plants off their dormancy cycles, causing loss of plants if they come out of dormancy in the middle of winter.
I concur with what MH stated. Go get a timer for $5 and all your problems are solved.